1. Field of the Invention
This invention relates to Magnetic Resonance Imaging, and more specifically, a head coil for producing a magnetic field gradient about a subject's head for Magnetic Resonance Imaging.
2. Description of Related Art
In nuclear magnetic resonance imaging, the magnetic field experienced by a proton in the sample determines the frequency at which that proton will re-radiate its acquired energy. Applying a magnetic field gradient which changes linearly with position over the sample effectively maps the re-radiated frequencies to a specific location in the sample. Non-linearity or inhomogeneity in the magnetic field gradient causes spatial distortion in the reconstructed image.
The subject to be imaged is immersed in a static magnetic field. Magnetic field gradients are produced by passing current through coils which encompass the subject to be imaged and provide a magnetic field which alters the static magnetic field. U.S. Pat. No. 4,646,024, Transverse Gradient Field Coils for Nuclear Magnetic Resonance Imaging issued Feb. 24, 1987 to Schenck, J. F., Hussain, M. A., Edelstein, W. A., and assigned to the instant assignee, describes the design of gradient coils. U.S. Pat. No. 3, 22,869, Homogeneity Coils for NMR Apparatus issued 1971 to M. J. E Golay also describes magnetic field gradient coils. These provide a magnetic field gradient over a subject's whole body.
Since it was determined that efficiency increases with smaller coils, in order to image the head, scaled versions of body coil assemblies were used. The efficiency of the coil was thereby increased but the design of the coil led to several problems. One such problem that resulted was that the image used some degree of spatial distortion due to poor linearity of the magnetic field gradient. Also, there were problems involving the introduction of artifacts re-radiation of the shoulders. That is, since the frequency of the reradiated signal is directly related to the magnetic field experienced by a resonating proton in the subject, regions of the subject within the same magnetic field intensity will radiate at the same frequency. Due to the geometry of the body coil, it is possible for more than one non-adjacent region of the subject to experience the same magnetic field. This causes an artifacts in the image due to the radiation of signals from two different parts of the body occuring at the same frequency thereby mapping to the same pixel.
In order to produce fast, high-quality images of a subject's head, a coil is required which can produce magnetic field gradients with a short rise time and a linear homogeneous gradient field.
There is a need for a magnetic gradient field coil for use in imaging a subject's head, which efficiently produces a strong gradient magnetic field having a high degree of spatial linearity. There is also a need for a head gradient coil which reduces or minimizes the problem of artifacts introduced by the subject's body.